Normally, chairs have been designed for style to decorate rooms according to period of fashion, mostly without regard for healthful comfort or functional behavior. A few "orthopedic" chairs have been designed with appropriate bracing features in the hopes of correcting malformed postures or curing backaches, but no chair can really do these things. Furthermore, when a sitter is forced by a chair's design into one "proper" posture, typically sitting up straight with both feet flat on the floor and prevented from assuming alternate or "unusual" postures, the resulting deprivation of motion inevitably leads to discomfort and pain.
With the foregoing rather narrow precept in mind, previous ergonomists have employed ratings of rising discomfort and have timed the onset of pain in subjects sitting up straight with fboth feet flat on the floor as discriminating criteria in evaluating features and dimensions of chairs. As a further criteria, these ergonomists have downgraded the value of a chair if subjects moved away from the "experimental" position and selected chairs in which subjects scored the least movements per unit timed. These investigators considered the number and intensity of movements, or "fidgeting", to be negative criteria because movement was seen to interfere with a primary activity such as reading (although reading scores were not registered), and movement was judged to be a response to pain.
To this day, no one has constructed a chair that does not discourage normal fidgeting and squirming and does not interfere with the myriad of postures and body-chair linkages the long term sitter involuntarily seeks to assume to escape the discomfort of sitting.
Some chairs have in the past been designed to relieve the discomfort and pain of one-position seating. For example, a vibrator has been attached to a chair to restore circulation. Other chairs use hinges and springs to allow the sitter to lean backward and then forward. Although these articulating chairs allow changes in the degree of hip flexion and some movement in the anterio-posterior plane, they are otherwise designed for one-position seating; twisting and turning movements are obstructed by these chairs. Rocking chairs provide for some movement in the knees and ankles only, but discourage gross posture changes or wide movements due to instability.
As a kinesiologist, and from my studies of the physiology of exercise, I know that most all postures are "proper" and "natural" and that movement from posture to posture (as one does in all forms of physical activity) is absolutely necessary for good body function, particularly for blood circulation, for well being, as motion releases excess tension and for well-doing as the kinesthetic stimulation of movement preserves alertness. Following this essential principle of salubrious movement, it is clear that any chair that traps the body into one position, as do "contoured" chairs with their seats dished or saddle-shaped, is unsound.